2017B Question 06

Describe the effects of morbid obesity on the respiratory system.

Examiner Report

52.2% of candidates achieved a pass in this question.

This question required a broad consideration of the physiological and anatomical parameters which change in the respiratory system of the morbidly obese. Candidates who scored well systematically outlined the various domains of the respiratory system, listed the changes seen and went on to describe the consequences and inter-relationship of these changes. In addition there were some marks allocated to the pathophysiological changes seen - as obesity is itself a pathology - however a sole focus on pathology was insufficient to achieve a pass. Notably there were no marks achieved for describing the metabolic, endocrine or cardiovascular effects of morbid obesity.

Model Answer

Structure:

  • Introduction
  • Anatomy
  • Lung volumes
  • Mechanics
  • Resistance and compliance
  • Ventilation and perfusion
  • Gas exchange
  • Disease: OSA, OHS

Introduction

Factor Detail
Definition - BMI >35kg/m2
Overall effect

- ↑ Resp demand

- ↓ Resp reserve

Pathophysiology

1. Mass effect (ME)

- Compression of neck, chest

 - Displacement of diaphragm

 - Worse if male (↑ central and visceral fat)

 - Better if female (↑ peripheral and subcut fat)


2. ↑ Metabolic rate (↑ MR)

- Due to ↑ muscle (~20% of extra mass) > ↑ fat (~80% of extra mass)


3. Adipokines from visceral fat (AK)

- Mostly pro-inflammatory: Leptin, TNFα, IL6, resistin, angiotensinogen, PAI-1

 - Some anti-inflammatory: Adiponectin

Anatomy

Airway region Detail
Upper airway

- ME → ↓ Radius → ↑ Resistance

- ME → ↓ ROM head and neck:

 - ↑ Risk OSA, OHS, obstruction when sedated

 - ↑ Risk difficult bag/mask ventilation and intubation

Lower airway

- ME → ↓ Lung volume → ↓ Airway radius → ↑ Resistance

- AK → Airway inflammation → ↑ Resistance:

 - ↑ Airway pressure

 - ↑ Risk gas trapping

Lung Volumes

Compliance Detail
Static

- ME → ↓ Chest wall compliance → ↓ TLC, RV, FRC (e.g. ↓ 25% if erect at BMI 30)

- Worse if supine, under GA

- If FRC < closing capacity: Small airway closure → ↑ Shunt:

  - ± Supine hypoxaemia

 - Rapid desaturation after induction

Dynamic

- ME → ↓ Chest wall compliance → ↓ FVC

- ME → ↑ Resistance → ↓ FEV1

- ↑ MR → ↑ VT 20%

Breathing Mechanics

Impediment Detail
Restriction

- ME → ↓ Chest wall compliance:

 - ↑ Work of breathing (WOB)

 - Severely impaired ventilation if Trendelenburg

Obstruction

- ME → ↓ Airway radius

- AK → Airway inflammation:

 - ↑ WOB

Alveolar Time Constants

Factor Detail
Definition

Resistance

- ↑ R due to ME, AK as above

Compliance

- ↓ C: Due to ME → ↓ Lung volume → ↓ Alveolar radius (

T

- ↑ Variability:

 - ↑ Slope phase 3 capnogram

 - ↑ Peak-plateau pressure difference

Ventilation and Perfusion

Factor Detail
V

- ↑ MR → ↑ RR 40%, ↑ TV 25%

- Accessory muscle use at rest:

 - ↑ WOB further

 - ↓ Reserve if unwell e.g. Pneumonia, exercise

Q

- ↑ MR → ↑ Cardiac output

- ± Risk OSA/OHS → Chronic ↓ PaO2 → +/- pulmonary hypertension:

 - Risk RV failure peri-op (e.g. Spont vent sedation → ↑ PaCO2 → ↑↑ PA pressure)

Gas Exchange

Factor Detail
↓ V/Q matching - FRC below closing capacity → Shunt
ABG changes

- ↓ PaO2 (if shunt)

- ↑ PaCO2 (if OHS)

- ↑ HCO3- (can be very high if OHS)

- pH usually normal if chronic process

Disease

Disease Detail
OSA

- ME + sleep → Airway collapse → ↓ PaO2 → ↑ SNS activity → Arousal → Repeat:

 - ↓ Sensitivity to ↓ PaO2

 - ↑ Risk obstruction and apnoea peri-op

 - ↑↑ Risk with opioids, benzodiazepines

 - 15% have pulmonary hypertension

OHS

- ME → ↑ WOB → ↓ VA → Chronic ↑ PaCO2:

 - ↓ Sensitivity to ↑ PaCO2

 - Reliant on hypoxic stimulus (ablated by volatile anaesthetics)

 - 50% have pulmonary hypertension


Last updated 2021-08-23

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